How to check if an object is an array?

Question

I\'m trying to write a function that either accepts a list of strings, or a single string. If it\'s a string, then I want to convert it to an array with just the one item so

Answer 1

I have updated the jsperf fiddle with two alternative methods as well as error checking.

It turns out that the method defining a constant value in the 'Object' and 'Array' prototypes is faster than any of the other methods. It is a somewhat surprising result.

/* Initialisation */
Object.prototype.isArray = function() {
  return false;
};
Array.prototype.isArray = function() {
  return true;
};
Object.prototype._isArray = false;
Array.prototype._isArray = true;

var arr = ["1", "2"];
var noarr = "1";

/* Method 1 (function) */
if (arr.isArray()) document.write("arr is an array according to function
");
if (!noarr.isArray()) document.write("noarr is not an array according to function
");
/* Method 2 (value) - **** FASTEST ***** */
if (arr._isArray) document.write("arr is an array according to member value
");
if (!noarr._isArray) document.write("noarr is not an array according to member value
");

These two methods do not work if the variable takes the undefined value, but they do work if you are certain that they have a value. With regards to checking with performance in mind if a value is an array or a single value, the second method looks like a valid fast method. It is slightly faster than 'instanceof' on Chrome, twice as fast as the second best method in Internet Explorer, Opera and Safari (on my machine).